The present invention relates to an imaging energy filter for electrons and other charged particles which filters an object formed by these particles at the filter inlet by means of an energetic selection of charged particles in the region of the dispersion aperture, due to the retransfer of the particles to original object locations which these particles have left, without loss of information about their positions.
The electro-optical effect of the additional element, which generates a field deflecting the charged particles by an angle of π−α/2, causes also the angle of the axes at the energy filter inlet and outlet relative to the primary axis to remain the same, and in consequence, the inlet and outlet axes are collinear.
The use of this element leads to a total deflection angle of 2π.
The invention relates also to possible uses of this imaging energy filter.
Imaging energy filters have a short history and have a relatively unassuming bibliography compared with methods of signal filters, for example retarding fields (the so-called “retarding field analyzer”) or with the classical electron spectroscopy, which is reflected in the work of H. Ibach, “Electron Spectroscopy for Surface Analysis” (Springer Publishing, 1977, Berlin).
In the first case, imaging with particles whose energies lie above a value determined with a grid potential (for example in low energy electron diffraction) is only possible incompletely, as the properties of the energy filters, which are used in laterally integrated electron spectroscopy measurement, do not permit direct adaptation for the benefit of two-dimensional image formation.
The first developments in spectroscopic imaging technology, which is based on the use of magnetic fields, are connected with electron transmission microscopy. Here, there are many innovative solutions, which have lead to mono-energetic images. The use of electrostatic filters in this case is made difficult because of the high energies of the electrons.
First, the dynamic development of emission and reflection electron microscopy (for example PEEM, LEEM) moved many new activities from the field of imaging energy filtration.
The combination of image formation and filtration technology, which is designated as spectromicroscopy, is desired here in particular, since in emission microscopy, as with the classical, optical microscopy, all object points are mapped in real time. In order to achieve a chemical contrast in this case also in real time, that is, the lateral distribution of different elements (assuming that the energy of the illumination of the probe is sufficient), one should select with the aid of the imaging energy filter a line that is characteristic for the specific element from the electron spectrum and minimizing the image-formation errors, visualize the probe with electrons originating from this narrow region of the energy spectrum.
The aspects connected with this topic are discussed in the work “The development of electron spectromicroscopy”, B. P. Tonner, et al, J. Electron Spectrosc. Relat. Phenom., 75 (1995) 309.
The plurality of such devices, which were discovered and made with the object of achieving a chemical contrast, uses the advantage of the hemispheric (180°) energy filter or its segments. This type of energy filter was first described by E. M. Purcell in Phys. Rev. 54 (1938) 8. Indeed, the image-formation properties as well as this filter were described, among other things, first in the work of B. Wannberg'a, G. Endahl'a I A. Skoellermo: “Imaging properties of electrostatic energy analyzers with toroidal fields”, J. Electron Spectr. Rel. Phenomen. 9 (1976), 111.
A special case of the filter mentioned in this article is the energy filter with cylindrical electrodes, which act on the charged particles only in one plane, which requires to the use of an additional electro-optical elements, which act only in the plane that is vertical to the first.
The new solution, which broadens the function of the imaging hemispherical filter, was proposed in U.S. Pat. No. 5,185,524. In the inner hemisphere, screened openings were made, through which electrons in the area of the retarding field penetrate and thereby improve the resolution of the system. The disadvantage of this invention is the reduction of the transmission as well as the increase of the electron underground from the secondary emission.
The combination of two filters, which are fragments of the spheres, was used in a spectromicroscope, which is described in U.S. Pat. No. 6,667,477 B2. Because of a symmetrical lens, which transmits the image from one filter to others, one obtains a reduction of the imaging error at the outlet of the entire system.
Since the charged particles in this system first are deflected at an angle α and then at an angle −α, the co-linearity of the axes on the system inlet and outlet is not fulfilled.
The imaging energy filter described in DE 69027602 T2 comprises four hemispherical energy filters, which are arranged in the shape of the Ω, such that the total angle amounts to 4π.
Although the particles going into and out of the energy filter move along the same axis, the system is complicated and has a compact structure, which makes difficult its construction and use. Nevertheless, the minimizing of imaging error is an advantage.
A similar solution is used in DE 19633496 A1.
The co-linearity of the inlet and outlet axes of the imaging energy filter and the total deflection angle of 3π was achieved also with the idea in DE 69705227 T2. The deflection of the particles takes place in two inclined planes: in one, twice at an angle π/2, and in the other, at 2π.
A similar solution, in which the combination of magnetic fields was used, can be found in DE 695 29987 T2. In this instrument, some variations of the deflection of particles are possible, which leads to the total deflection at 2π and the overlapping of the inlet and outlet axes.
One of the possibilities is the double deflection of the particles in a first sector of the magnetic field at an angle of π/2 and the single deflection at an angle of π in a second sector of the magnetic field.
In all of these variations of the system, the deflection takes place in a plane that is common to all of the elements.